Open Source,Hardware DIY Reflected Light Meter

I've used this old project as a source, it was an incident one, and I made a reflected one (doesn't have a dome).

My main differences are the use of a TSL2591 instead of TSL2561, a RED/GREEN LED rotary encoder, a BL-5C battery, and a PCB to fit everything, and a whole firmware rewrite.

My project sources are here.

I've choosed a BL-5C battery because they are cheap, and easy to find, and doesn't need a whole disassembly to swap.

Calibration

I've calibrated the light meter only inside for now, with a studio lamp and 18% grey card, it seems ok, with a real-life test the pictures from the DSLR in manual seemed a little bit over-exposed or under depending on some conditions.

I will do two other calibration tests outside, the first results and method are documented here.

I don't really know if my calibration method is right, but I have been unable to find anything related to reflected light meter calibration, only for incident or flash meters.